1. Field of the Invention
The present invention relates to a transducer having a bolted Langevin type vibrator, and more particularly to an acoustic transducer which is capable of obtaining a sound pressure level equal to or higher than a given level over a wide frequency band.
2. Description of the Related Art
At present, a miniature bolted Langevin type sonic transducer which can transmit a high power sound wave is used as an acoustic transducer for use in water. FIG. 1 shows a cross sectional view of an example of a conventional bolted Langevin type sonic transducer disclosed in JP 3,406,986 B. The sonic transducer includes a bending vibrator 2 and a Langevin type vibrator 3. The bending vibrator 2 has a disc type active diaphragm 4 and a diaphragm 5 having a cavity in its inside. The diaphragms 4 and 5 are bonded to each other through an adhesive agent. The Langevin type vibrator 3 has a cylinder type active vibrator 9, a rear mass 7, a front mass 6, and a bolt 8. The front mass 6, a plurality of cylinder type active vibrators 9 and the rear mass 7 are provided in tandem. The bolt 8 tightens those constituent elements. The disc type active diaphragm 4 is located within a concave portion in a front face of the front mass 6. The diaphragm 5 is welded to the front mass 6 through a joining portion 60. When the disc type active diaphragm 4 vibrates, the diaphragm 5 vibrates to radiate a sound wave. In addition, the transducer includes a phase shifter 10 for shifting a phase of a driving voltage applied to the disc type active diaphragm 4.
The above-mentioned transducer has three vibration modes: a bending vibration mode of the bending vibration 2 at a frequency fa; a longitudinal vibration mode of the Langevin type vibrator 3 at a frequency fb; and a bending vibration mode of a front face plate (an uppermost portion in FIG. 1) of the vibrator 5 at a frequency fc. Normally, those frequencies show a relationship of fa<fb<fc. The transducer is driven so that the longitudinal vibration mode of the Langevin type vibrator 3 becomes opposite in phase with respect to the bending vibration mode of the bending vibrator 2. Thus, the vibration modes are superposed, and the transducer operates at a frequency of a wide frequency band.
However, this transducer involves the following problem. FIGS. 2A to 2C shows examples of vibrations of the transducer. When a size of the transducer is not changed, a sound pressure level in a first resonance frequency f1′ based on the bending vibration mode (FIG. 2A) of the bending vibrator 2 is low in a low frequency region because an area of the bending vibration 2 is small in the low frequency region with respect to a wavelength thereof. Moreover, the sound pressure level is remarkably reduced in an intermediate frequency f′ region between a second resonance frequency f2′ based on the longitudinal vibration mode (FIG. 2C) of the Langevin type vibrator 3 and the first resonance frequency f1′. This reason resides in that since the Langevin type vibrator 3 and the bending vibrator 2 are integrated with each other, the vibration modes of the Langevin type vibrator 3. and the bending vibrator 2 are coupled to each other. As shown in FIG. 2B, a sound radiation area (an area α in FIG. 2B) due to the bending vibration mode of the bending vibrator 2 and a sound radiation area (an area β in FIG. 2B) due to the longitudinal vibration mode of the Langevin type vibrator 3 vibrate in directions opposite to each other to cancel the sound pressures of the area α and the area β.